1. Field of the Invention
The present invention relates to a zoom lens. The present invention further relates to an image pickup apparatus such as a digital camera equipped with a zoom lens.
2. Description of the Related Art
In recent years, digital cameras have replaced film cameras to become the mainstream in the technical field of the image pickup apparatus. Digital cameras have image pickup areas that are generally smaller in size than those of film cameras, and therefore a reduction in the size of digital cameras can be achieved more easily. Nowadays, since there is the trend that a higher importance is placed on the portability as compared to the past, the sizes of digital cameras have been made smaller.
Furthermore, since users of digital cameras wish to enjoy easy shooting in both indoor and outdoor shooting situations, increases in the angle of view and increases in the zoom ratio of zoom lenses used as the optical systems of digital cameras are needed. Various zoom lens have been developed in the past with a view to achieve a smallness in size, a wide angle of view, and a high zoom ratio.
For example, Japanese Patent Application Laid-Open Nos. 2008-46529 and 2007-179015 disclose negative-front type zoom lenses that have a lens unit having a negative refracting power disposed closest to the object side. This type of zoom lens is advantageous in achieving a large angle of view at the wide angle end, and advantageous also in reducing the overall size of the zoom lens because the size of the lens unit closest to the object side can be made small.
On the other hand, in order to achieve a further increase in the zoom ratio, a further inventive design is needed. In the cases of the zoom lenses disclosed in the aforementioned Japanese Patent Application Laid-Open Nos. 2008-46529 and 2007-179015, the refractive index of the positive lens in the first lens unit is low, and it cannot provide a sufficiently high positive refracting power. Otherwise, the positive, lens in the first lens unit may need to have a high refractive index, which requires use of a material, such as diamond, that is low in availability and very hard to process.
Furthermore, since the negative lens in the first lens unit in these zoom lenses has a meniscus shape, if a further increase in the zoom ratio and/or angle of view is to be achieved, one surface of the negative lens in the first lens unit needs to have a strong negative refracting power, which is likely to lead particularly to increases in off-axis aberrations.